The present invention relates to a process and device for contacting the surface treatment of a strip material with a liquid. More particularly, the invention is directed to a trough for treating a rolled metal strip with a pickling solution.
According to the state of the art, strip processing plants, such as those used for metal pickling plants, using liquid baths are designed with each individual bath measuring approximately 100 to 200 mm deep and normally 10 to approximately 40 m long. The reaction speed of the treatment medium on the surface of the strip material being processed is improved by reducing the liquid boundary layer on the strip surface. To reduce the liquid boundary layer, the current trend is now towards shallower liquid baths. This leads to greater bath turbulence due to the higher Reynolds number and a reduction in the liquid boundary layer due to the increased shearing effect.
The lower limit of the average depth of the bath is dependent on the speed of the strip material passing through the bath. Due to the dragging effect of the moving strip, the liquid treatment medium is carried along in the treatment tank in the same direction as the advancing direction of the strip. This results in a slant or incline of the liquid surface level with a corresponding increase in the depth of the bath at the strip exit point and the equivalent reduction in the depth of the bath at the strip entry point. At higher speeds (e.g., above a strip speed of approximately 200 m/min with 200 mm mean depth and 20 m length of the bath), the liquid is carried away by suction from the strip entry point into the treatment bath such that the distance over which the strip is submerged in the treatment medium is shorted accordingly. This results in the length of the treatment bath and the tank not being fully utilized. As a result, the mean liquid level of the bath must be raised to avoid the strip entry area being emptied by the suction from the advancing strip. The optimum efficiency of a treatment bath with an open top surface is thus a compromise between the shortened bath length as a result of the draft effect and the reduced pickling effect because of the greater mean depth of the treating liquid bath.
One example of a bath having a structure to maintain a small mean bath depth at high speeds is by the use of a closed treatment channel. The depth of the bath is thus limited to the height of the treatment channel. In order to prevent the liquid from being drawn or sucked out of the treatment channel, the strip exit point from the treatment channel must be sealed hydrodynamically against the back-up pressure of the dragging effect. An example of this type of structure and process is disclosed in, for example, EP 0 655 519 A1. The hydrodynamic seal requires a much higher energy input compared to the treatment baths with an open top surface. Furthermore, the high velocity and head pressure caused by being sealing off at the strip exit point of the treatment channel makes it more difficult to provide the required supply of fresh treatment medium. The closed channel requires more energy to feed the fresh liquid treatment medium into the channel. Formation of a closed channel reduces evaporation of the liquid treatment medium and thus reduces the amount of energy needed to maintain the treatment temperature. This, however, it not an advantage, particularly in pickling plants which recover the acid by heat decomposition using additional equipment. The quantity of liquid that is not evaporated in the processing tank must be removed by other processes thereby increasing the energy consumption for recovery of the acid.
Accordingly, there is a continuing need in the industry for an improved process and apparatus for treating a moving strip in a liquid bath.
A primary object of this invention is to improve the efficiency of a treatment bath with an open top surface thereby avoiding the disadvantages of a closed treatment channel.
A further object of the invention is to provide a panel mounted above a strip being treated and submerged in the liquid in a treatment bath which divides the treatment bath into a treatment area underneath the panel and a return flow sector for the liquid above the panel.
Another object of the invention is to provide a process and apparatus for returning the liquid from the discharge end of a treatment bath tank to a strip material receiving end of the tank.
A further aspect of the invention is to provide a trough containing a liquid for treating a sheet material where the trough has a small bath depth.
Another aspect of the invention is to provide a trough containing a liquid treating bath for treating a sheet material where the movement of the sheet material through the treating path produces mixing of the bath and circulates the bath from the one end of the other.
A further aspect of the invention is to provide a trough containing a liquid for treating a sheet material having a divider defining a treating zone and a liquid return zone, and having openings in the divider to provide liquid communication between the treating and return zones.
Another aspect of the invention is to provide a trough containing a liquid for treating a sheet material having a divider forming a treating zone and a liquid return zone where the treating and return zones are in fluid communication at a receiving end and a discharge end of the trough.
Another object of the invention is to provide a trough for contacting a continuously moving sheet material through a liquid bath using a small bath depth.
A further object of the invention is to provide a trough for contacting a continuously moving sheet material with a liquid bath in a manner to maximize the usable length of the trough.
The objects of the invention are basically attained by providing an apparatus for immersing a strip material in a liquid bath, the apparatus comprising: a trough having a longitudinal dimension with a bottom wall, first and second side walls, a strip material receiving end and a strip material discharge end, the trough being dimensioned to receive a treating liquid and contain a liquid bath; and at least one panel mounted within the trough and submerged in the liquid bath, the at least one panel having a bottom surface spaced from the bottom wall of the trough for defining a contact zone in the trough, the at least one panel further having a top surface defining a return zone for receiving liquid from the discharge end of the trough and directing the liquid along the top surface from the discharge end to the receiving end of the trough.
The objects of the invention are also attained by providing an apparatus for immersing a strip material in a liquid bath, the apparatus comprising: a trough containing a liquid bath, the trough having a longitudinal dimension, first and second side walls, a bottom wall, and a first end wall at a strip material receiving end and a second end wall at a discharge end for the strip material; a divider positioned in the trough defining a contact zone and a liquid return zone for directing liquid from the discharge end to the receiving end of the trough; and a feed device for feeding strip material from the receiving end to the discharge end through the contact zone.
The objects of the invention are further attained by providing a process for contacting a strip material with a liquid bath, comprising the steps of: feeding a strip material to a receiving end to a discharge end of a trough, wherein the trough contains a liquid bath between the receiving and discharge ends, the trough including a divider defining a strip material contact zone in a bottom section of the trough and a liquid return zone in a top section of the trough; conveying the strip material through the contact zone and creating a current of the liquid through the contacting zone from the receiving end to the discharge end; directing the current of liquid at the discharge end to the return zone and returning the liquid to the receiving end; and discharging the material strip from the discharge end of the trough.
These and other objects of the invention will become apparent from the detailed description of the invention which taken in conjunction with the annexed drawings disclose preferred and various embodiments of the invention.